Magnetoelectronics, spin electronics, and spintronics are synonymous terms for the use of effects predominantly caused by electron spin. Magnetoelectronics is used in numerous information devices, and provides non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, magnetic random access memory (MRAM), magnetic sensors and read heads for disk drives.
Typically, a magnetoelectronic device, such as a magnetic memory element, has a structure that includes multiple ferromagnetic layers separated by at least one non-magnetic layer. In a memory element, information is stored as directions of magnetization vectors in the magnetic layers. Magnetization vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch between the same and opposite directions that are called “parallel” and “antiparallel” states, respectively. In response to parallel and antiparallel states, the magnetic memory element represents two different resistances. The resistance has minimum and maximum values when the magnetization vectors of the two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of change in resistance allows a device, such as an MRAM device, to provide information stored in the magnetic memory element. The difference between the minimum and maximum resistance values, divided by the minimum resistance is known as the magnetoresistance ratio (MR).
The physical structure of these magnetic elements typically includes thin layers, some of which are in the range of tens of angstroms thick. The performance of the magnetic element is also relatively sensitive to the surface conditions on which the magnetic layers are deposited. Accordingly, it is generally desirable to make as flat a surface as possible in order to prevent the operational characteristics of a magnetic element from exhibiting undesirable characteristics.
During typical magnetic element fabrication, such as MRAM element fabrication, which includes metal films grown by sputter deposition, evaporation, or epitaxy techniques, the film surfaces are not absolutely flat but instead tend to exhibit some surface or interface roughness. This roughness of the surfaces and/or interfaces of the ferromagnetic layers can be a source of magnetic coupling between the free ferromagnetic layer and the other ferromagnetic layers, such as the fixed layer or pinned layer. This magnetic coupling is commonly known as “topological coupling” or “Néel's orange peel coupling.” Such coupling is typically undesirable in magnetic elements because it can create an offset in the response of the free layer to an external magnetic field. Additionally, the roughness may also introduce a certain amount of degradation in the electrical characteristics of the device by scattering conduction electrons or by causing variations in the tunneling current of the tunnel junction.
A magnetic structure is known as bottom pinned when the fixed layer is formed before the spacer layer, and the free layer is formed after the spacer layer. In such a bottom-pinned structure the antiferromagnetic (AF) pinning layer is contained in the bottom magnetic electrode. Conventional bottom-pinned magnetic tunnel junctions (MTJs) and spin valve structures typically use seed and template layers to produce an oriented, crystalline AF layer for strong pinning.
The bottom electrode of a typical bottom-pinned MTJ structure includes stacked layers of tantalum, a nickel iron alloy, iridium manganese, and a cobalt iron alloy (Ta/NiFe/IrMn/CoFe), which is generally followed by an aluminum oxide (AlOx) tunnel barrier, and a top electrode that typically includes a free layer of nickel iron (NiFe), where the tantalum nitride iron (Ta/NiFe) seed/template layers induce growth of a highly oriented iridium manganese (IrMn) layer. This highly oriented IrMn layer provides for strong pinning of the CoFe layer below the AlOx tunnel barrier. However, the IrMn layer, or other similarly oriented polycrystalline AF layer, typically produces a roughness that can cause an increase in the undesirable Néel coupling between the pinned CoFe layer and the top free NiFe layer, as well as other undesirable electrical characteristics.
In practical MTJ elements, the bottom electrode is generally formed upon a base metal layer that provides a relatively low resistance contact to the junction. The base metal layer is typically polycrystalline and produces a roughness that, in turn, propagates into the bottom electrode and can also produce roughness at the spacer layer interfaces, resulting in an increase in undesirable Néel coupling between the pinned CoFe layer and the top free NiFe layer. The roughness, propagated from the base metal layer and the bottom electrode, is additionally undesirable because it can limit the minimum tunnel barrier thickness that can be achieved while retaining high MR and device resistance that scales inversely with the junction area. Accordingly, it is generally desirable to reduce the surface roughness of the various layers, where possible to do so.
This desire to reduce the roughness of the layers and the layer interfaces has led to use of non-crystalline or amorphous materials in various layers of a multi-layer MTJ stack. Since the amorphous materials typically lack the crystal boundaries and sharp features of other materials, the tunnel barrier resulting from the layers with the amorphous materials will typically provide for enhanced device performance. However, in addition to the advantageous properties useful in forming tunnel barriers, many amorphous materials also exhibit certain undesirable characteristics as well. Specifically, most amorphous alloys exhibit at least one undesirable property such as low recrystallization temperature, low MR, high dispersion, high magnetostriction, or unstable anisotropy. Depending on the desired performance characteristics of the specific magnetoresistive elements, some of these characteristics may result in devices with relatively poor performance.
Accordingly, it is desirable to provide materials which not only reduce the surface roughness of the various layers that form the MTJ elements, but that do not also introduce negative performance factors into the resulting magnetoelectronic devices. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.